Whenever an ink drop is ejected from an orifice of an orifice plate, a trailing portion or "tail" of ink moves with the drop. A small amount of the ink tail may separate and land on the outer surface of the plate as an ink droplet. Residual ink that collects on the orifice plate outer surface near the edges of the orifices may contact subsequently ejected ink drops, thereby altering the trajectory of those drops, which reduces the quality of the printed image.
Also, in the event that a substantial amount of residual ink accumulates on the orifice plate outer surface, a continuous liquid path between the ink within the orifice and the ink on the outer surface may be formed, thereby facilitating leakage of the ink out of the orifice. Further, when a substantial amount of ink accumulates on an orifice plate, this large pool of ink can interfere with drop ejection to the extent that no drops are ejected, i.e. a single drop is unable to pass through the large pool of ink. Moreover, the residual ink on the outer surface of the plate tends to trap minute particles, such as paper fibers, thereby interfering with the trajectory of subsequently-ejected drops.
The inner surface of the orifice plate is in contact with ink and modifying the surface energy of this portion of the plate is an advantage to optimize the performance of the pen, i.e. fine tuning the characteristics of the ink drops as they are ejected from an orifice.
As discussed in more detail in prior application (Ser. No. 07/724,648, filed Jul. 21, 1991, European Patent Application No. 0 521 697 A2), it has been found that the wetting characteristics of certain portions of the outer surface of an orifice plate can be made to enhance pen performance. More specifically, that application describes various techniques to make the outer surface of the orifice plate immediately surrounding the orifices nonwetting with respect to the ink. As a result, it was found that any residual ink deposited on the outer surface of the plate in those regions would bead up away from the edges of the orifices so as not to interfere with the subsequent ejection of ink drops. The remaining portion of the outer surface of the orifice plate is made wetting with respect to the ink such that any residual ink which comes into contact with this portion will flow off the plate under the influence of gravity, or by wipers. Further, as also described in that application the inside surface of the plate can be treated to be a wetting surface with respect to the ink in order to facilitate the flow of ink into and through the orifices.
As described in the referenced application, the orifice plate was made wetting or nonwetting by techniques such as plasma etching or reactive ion etching. Alternatively, a nonwetting surface could be made by a spray-application of a cross-linked silicone resin, such as methyltrimethoxy silane manufactured by Dow Corning and designated QI-2645. The nonwetting characteristics of the orifice plate surface, as described in the referenced patent application, could be made nonwetting by the application of a fluorocarbon or silicon polymer layer via a conventional plasma polymerization technique.
While these techniques are more than satisfactory for overcoming the problems associated with ink-jet pen performance identified in that application, the present invention provides an orifice plate (and methods for making the same) having significant differences in the chemistry of its makeup, its construction, and the manner by which it functions to affect the overall printing quality and performance of the pen.
More specifically, the present invention is directed to the chemisorption or chemical bonding of certain compounds, such as thiols, to form self-assembled monolayers on the orifice plate. Self-assembly is the spontaneous adsorption of thiols or other compounds on a solid surface from solution or other direct molecular contact which results in a monolayer of oriented molecules on the metal surface. For example, for an alkyl thiol HS(CH.sub.2).sub.n X, the sulfur is bonded to the gold and the terminal group X is at the surface of the monolayer, i.e. monolayer/air interface. The terminal functional group X renders the orifice plate either wetting or nonwetting depending on the terminal group selected. The present invention is also advantageous in that the monolayers are easily applied in a relatively short period of time.
Further, the present invention provides an orifice plate having an inner surface treated with a self-assembled monolayer as mentioned above. It has been found that treating the inner surface with a monolayer of certain compounds affects the rate at which the firing chamber refills after a drop is ejected. In particular, by treating the inner surface with a self-assembled monolayer, it is possible to obtain a constant or steady state ink drop weight at a specific frequency, or the rate at which the ink drops are fired. As should be appreciated, a constant ink drop weight is critical to the overall printing quality and performance of the ink-jet pen. In addition, obtaining a constant ink drop weight at a specific or higher frequency allows for greater control over the performance of the pen. Importantly, it has been discovered that such results can be obtained by using either wetting or nonwetting monolayers on the inner surface of the orifice plate.
In addition, treating an orifice plate with a monolayer inhibits deterioration and contamination of the orifice plate. Consequently, better printing quality over the life of a pen is achieved.